This invention relates to seal wire heat control systems and, more particularly, to seal wire heat control systems for sealing plastic films used in plastic packaging.
For the past 40 or 50 years, hot wires have been used to make seals when sealing plastics, particularly plastic films such as those used in plastic packaging, especially shrink packaging. In some cases, the hot wires are used to merely make a seal in the edge of the film, but more frequently they are used to seal and sever the edge of a pouch or bag from the succeeding pouch and/or from a selvedge edge of the film. In a typical application two layers of film, one superimposed on the other, will be sealed together by means of a hot wire pressing against the film and trapping it between the wire and an elastomer bed. The hot wire will simultaneously seal the edges of the film together on both sides of the wire while severing, by melting and/or softening, the film trapped between the wire and the elastomeric bed.
For many years control of the temperature of the wire has been a major problem in the industry. If the wire runs too cool, it will not make a proper seal and cut in the thermoplastic film. If the wire runs too hot, it may destroy the seals adjacent to the wire by overheating them, and the excessive temperature will cause some films to stick to the wire and gum up. Some films, such as polyethylene, will smoke, and films such as PVC will give off fumes and form a black carbonaceous residue on the seal wire if the wire gets too hot. Thus, seal wire temperature control is extremely important.
Known sealing systems have suffered from the foregoing problems. For example, U.S. Pat. Nos. 3,490,981 and Re 30,010 to F. G. Shanklin show an L-Sealer machine using hot wire seals such that two seal wires are brought together at a corner to form an "L" U.S. Pat. No. 4,219,988 to Shanklin et al shows a Form-Fill-Seal machine in which a single seal wire makes the transverse seals. Both types of prior art machines encounter similar problems in controlling the seal wire temperature.
Temperature control of the wire, in prior art devices, was accomplished with several types of systems, each of which was prone to inaccuracies of control. The earliest form of such wire temperature control was an impulse hot wire seal in which voltage was imposed across a length of a resistance wire for a period of time that was predetermined by a timer. The wire would heat to a certain temperature and then shut off. As long as packaging speeds were slow (10-12 packages per minute), this system seemed to work quite well. As soon as speeds increased, the seal jaw would heat up and the wire would build up residual heat as the machine continued to run. The seal wire and jaw would gradually overheat with the resultant problems of poor seals and gummed up wires.
The next step in the process of improving wire temperature control was to add a wire temperature switch, sometimes called a compensator switch, to the seal wire system. The wire temperature switch consisted of a limit switch or set of electrical contacts mounted on an adjustable base, which was positioned so that the switch would be operated by a wire follower as the seal wire expanded. Customarily, seal wires have one fixed end and one movable end, and tension in the seal wire is maintained by a spring attached to a wire follower to which is attached the movable end of the seal wire. As the seal wire expands the follower will move, driven by the spring. At a predetermined temperature setting, the wire will have expanded a certain amount allowing the wire follower to move until it actuates the limit switch turning off the power to the wire. A low voltage current across the wire can be maintained at all times to prevent the wire from returning to room temperature. This shortens the time cycle required to make a first seal and results in a more uniform operation of the sealer.
With all the above mentioned systems, as the machine ran faster and more power was input to the seal wire, residual heat would build up in the seal wire and the mating beds, and, notwithstanding the wire temperature switch control, the machine would run hot. Such excessive heat would cause smoking, gumming, and deterioration of the seal.
An additional problem occurred with the above described system. Since a system with a wire temperature switch (or compensator switch) depends on measuring the length of the seal wire to determine the seal wire heat, the distance between the fixed end and movable end of the wire must remain constant. Any increase in the length of the arm upon which the switch was mounted due to thermal expansion would also add to the temperature of the seal wire. In fact, the seal wire would increase in temperature by an amount equal to the increase in temperature of the seal arm multiplied by the coefficient of expansion of the seal arm and divided by the coefficient of expansion of the seal wire. Thus, since the most common construction for seal arms is aluminum with a coefficient of expansion of 13.times.10.sup.-6 and the seal wires are made of a nickel, chromium, iron alloy with a coefficient of expansion of approximately 8.times.10.sup.-6, one degree rise in the temperature of the seal arm will result in 1.6 degrees temperature rise of the seal wire. It is not unreasonable to expect a temperature rise in the seal arm of at least 70.degree. F. which will cause a 112.degree. F. increase in temperature of the seal wire. This is in addition to the temperature increase from increased residual heat accumulating in the wire and beds. It can be seen from the foregoing that it is not possible to accurately control the seal wire temperature by measuring the length of the seal wire with a device that is mounted on a seal arm which gets hot from use.
Other prior art devices measured the temperature of the seal wire by measuring the resistance of the wire between impulses of heat. Thus, a device can be built that will impose a brief pulse of power to a seal wire, disconnect the power, and measure the resistance of the wire, then resume applying power to the wire, then again measure the resistance, etc. until the resistance measured corresponds to the desired seal wire temperature. Although these systems have been built, they suffer from un-reliability One reason for this is that the resistance of the seal wire is already very low, on the order of 2 ohms for a 60" length of a typical seal wire, and the rate at which the resistance of the seal wire changes in relation to the temperature of the wire is very small. Thus, any resistance that is entered into the system, such as by a poor wire connection, moisture, corrosion, etc., will have considerable effect on the control of the seal wire resulting in erratic temperatures. A further disadvantage to this prior art system is that it has been very expensive.
In summary, three systems have been used in the prior art for determining and controlling a seal wire's temperature. The first system imposed a voltage across the seal wire for a period of time that was predetermined by a timer. This system neglects the effect jaw parts due to residual heat from previous seals. The second system measures the length of the seal wire as it expands with increased temperature. This system is faced with the inaccuracies that occur from elongation of the seal arm caused by thermal expansion of the seal arm as it heats up due to previous seals. The third system measures the resistance of the seal wire and, by using resistance wire with a proper resistance to temperature gradient, translates the resistance into seal wire temperature and then controls the power to the wire. This system runs into errors occurring from trying to accurately measure extremely small resistance changes in a very low resistance wire and from other resistances that can occur anywhere in the seal wire circuit.